UNH Scientists Report First Findings On Key Astrophysics Problem

By David Sims, Institute for the Study of Earth, Oceans, and Space November 28, 2007

In a paper published recently in the journal Nature Physics, an international
team of space scientists led by researchers from UNH present findings on the
first experimental evidence that points in a new direction toward the solution
of a longstanding, central problem of plasma astrophysics and space physics.

The mystery involves electron acceleration during magnetic explosions that
occur, for example, in solar flares and “substorms” in the Earth’s
magnetosphere – the comet-shaped protective sheath that surrounds the
planet and where brilliant auroras occur.

During solar flares, accelerated electrons take away up to 50 percent of the
total released flare energy. How so many electrons are accelerated to such
high energies during these explosive events in our local part of the universe
has remained unexplained.

A mainstream theory holds that the mysterious, fast-moving electrons are primarily
accelerated at the magnetic explosion site – called the reconnection
layer – where the magnetic fields are annihilated and the magnetic energy
is rapidly released. However, physicist Li-Jen Chen of the Space Science Center
within the UNH Institute for the Study of Earth, Oceans, and Space discovered
that the most powerful electron acceleration occurs in the regions between
adjacent reconnection layers, in structures called magnetic islands.

When Chen analyzed 2001 data from the four-spacecraft Cluster satellite mission,
which has been studying various aspects of Earth’s magnetosphere, she
found a series of reconnection layers and islands that were formed due to magnetic
reconnection.

“Our research demonstrates for the first time that energetic electrons
are found most abundantly at sites of compressed density within islands,” reports
Chen.

Another recent theory, published in the journal Nature, has suggested that “contracting
magnetic islands” provide a mechanism for electron acceleration. While
the theory appears relevant, it needs to be developed further and tested by
computer simulations and experiments, according to the authors.

Until the UNH discovery there had been no evidence showing any association
between energetic electrons and magnetic islands. This lack of data is likely
due to the fact that encounters of spacecraft with active magnetic explosion
sites are rare and, if they do occur, there is insufficient time resolution
of the data to resolve island structures.

In the Nature Physics paper, entitled “Observation of energetic electrons
within magnetic islands,” lead author Chen reports the first experimental
evidence for the one-to-one correspondence between multiple magnetic islands
and energetic electron bursts during reconnection in the Earth’s magnetosphere.

“Our study is an important step towards solving the mystery of electron
acceleration during magnetic reconnection and points out a clear path for future
progress to be made,” says Chen.